Modifications to a Philips transceiver model FM-92 to convert its operation from commercial VHF low band (70-85 MHz) to 6 metre HAM band
There are several versions of the FM-92, for high Band VHF and UHF, this page primarily covers the 99 channel low Band VHF,
25 watt output ( E band, 66-88 Mhz model)
The FM-92 is part of the Philips FM-900 series, see also FM-91 and FM-93 models. It comes in E, A, T, U, W1 & W2 band versions.
This transceiver is EPROM programmable, normally a 2764 (8kbyte) EPROM.

The FM-92 is characterised as a mobile base unit, it comes into two variations, a "Local" unit, with front panel fixed to the base unit and
the "Remote" unit, which has the front, control panel connected to the main unit by a remote cable with DB-15 connectors at each end
approx 25cm L, 20cm W, 6cm D, with a diecast body with heatsink ribbing and sides and rear panel.
connectors are a 15 pin DB-15 for remote head connection, a BNC antenna socket and 13.8V DC power lead.
The remote Head and Local front panel, includes twin 7 segment displays for channel indication and several buttons for UP/DOWN channel
selection, SEND button, AUX button, POWER on/off button, plus Volume and Mute controls, Power, TX & RX indication LEDS.

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FM92-E conversion to the Amateur 6 metre band (52-54 Mhz) by CCARC
These are the main two documents relating to the conversion procedures developed by CCARC,
very detailed with colour photos and looks to be the best approach (not that I have started yet)

check Rod VK2DOT's site for latest updates to his documentation and EPROM files
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MODIFICATIONS FOR PHILIPS FM92 A BAND (148-174MHz) TO THE AMATEUR 2 metre BAND (144-148 MHz)
First :- BE GENTLE - DO NOT FORCE TUNING SLUGS - they break easily. If this happens, try removing them by unscrewing
from the other end of the former. Use a PROPER plastic tuning tool or they WILL break !!!
1. check to see radio works on original frequencies (printed on label inside cover), with dummy load! Should get
noise on open squelch, and 25 watts out.
2. replace EPROM. If creating a job with FM900 software, change the hardware ID number to the lower frequency range
(135-148Mhz ?), otherwise it won't accept our frequencies.
3. tune VCO slugs -
(2 slugs located inside a large can, nearest the side of the set. Early versions can only be adjusted from the
component side, later ones from the track side as well.)
Pick a mid frequency channel, tune slugs - one for VCO transmit freq., one for VCO Receive freq.. Monitor the VCO
control voltage at TP201 on the receive PCB (just near the Audio amp IC with the heat sink tag). It will be stuck
at maximum voltage before the adjustment.
When adjusted it should come down to about 8 to10 volts. Adjust the Rx slug first, then pushing the PTT adjust the
TX slug. Try to adjust both so that you get the same voltage (about 10 volts ?) when switching between RX and TX
mode on a simplex channel. Check that no channels give a voltage outside the 4 - 10 volt range - they may go out
of PLL lock.
4. TX should now transmit on frequency - tweak the only trimmer cap on the PA PCB for maximum power out (usually
fully meshed).
5. With a weak signal source attached to antenna socket, adjust the 8 slugs in the receiver front end (in the big
can located in the center of the RX PCB). Start by winding each slug in about 3 turns. Adjust for minimum noise.
Local the 'S' signal level test point and adjust slugs again several times for maximum level. You may want to
'stager' tune the front end for broad band operation (tune some for a mid-high channel , some for a mid-low channel).
Notes:
A. Modulation on some sets is not good - very 'tinny' - too much high frequency pre-emphasis. (see fixes below)
B. Scanning (push the AUX button) only works when the microphone is in it's cradle (magnet activates reed switch
in mic.).
C. Changing appropriate link on the synth PCB gives 12.5KHz frequency steps(but still 25Khz bandwidth) but requires
appropriate EPROM. FPP/FM900 software assumes the links have been changed if you accidently enter a channel with
a 12.5K step and doesn't warn you (and wont work until the link is changed ! ).
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FM900 12 Digit Hardware codes
A 12-Digit Product Code Is Derived From The Chart Below The Firs 4 Digits (9502) Denote Austrilia And Are Fixed.
The Remaining 8 Digits of the code detail the type of Equipment And Frequency Band (center 3 Digits- Colums 5,6,7),
And Optional HardWare Facilities (last 5 Digits - Colums 8 Thru 12)
1 2 3 4
9 5 0 2
5 6 7 8 9 10 11 12
Series Model Freq/Band Local/Remote Temp/Range Signaling Selcall Option
Spacing Stability Hardware System
--------------------------------------------------------------------------------------
0 10 ppm STD STD STD
Standard (none) No S/Call
--------------------------------------------------------------------------------------
1 5ppm CTSS CCIR Rev/Tone
Xtal/Heater Enc Only Burst
--------------------------------------------------------------------------------------
2 SimPlex E CTSS ZVIE
ENC/DEC
--------------------------------------------------------------------------------------
3 B Local SelCall EEA
Type 1
20/25/30 No CTSS
--------------------------------------------------------------------------------------
4 A Remote CTSS ENC+ EIA Ignition
20/25/30 SelCall T1 Blanker
--------------------------------------------------------------------------------------
5 T 2.5ppm CTSS ENC/DEC I/O Exp
TXCO Kit SelCall T1 No S/Call
--------------------------------------------------------------------------------------
6 U SelCall CCIR T2
Type 2 ENC Only
No CTSS
--------------------------------------------------------------------------------------
7 W1 12.5 Local CTSS ENC ZVEI T2
SelCall T2 ENC Only
---------------------------------------------------------------------------------------
8 W2 12.5 Remote CTSS ENC/DEC EEA T2
SelCall T2 Enc Only
---------------------------------------------------------------------------------------
9 Mobile EIA T2
ENC Only
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FM-900 noisy VCO fault
Below is an extract from APCNews 16 August 2000, regarding the FM900 noisy VCO fault repair procedure...
Dear Dr Fixit,
I have just bought an FM900 at the local Hamfest, and when transmitting, other amateurs report low audio and a rumbling
or rushing noise on my transmission, also a rumbling can be heard in Receive mode, can you please help?
Answer:
Well, Rumbling old chap, it seems that you have the common "Noisy VCO" found in many of the FM900 series radios.
The best way to test this is to listen to a fully quieting signal in receive mode, with no modulation, then turn up the
volume, and tap the main body of the radio with your knuckle, screwdriver handle or similar and listen to the received
signal. You should not hear any significance sound from the speaker. If you do , it's likely that you have a noisy or
microphonic VCO.
This is caused by the ceramic substrate in the VCO de-laminating from the metal housing, any small vibrations cause
capacitance changes within the VCO module, and so you "hear" these vibrations in the receiver as audio.
On the TX side its basically the same problem, symptoms and cure. Using the same tapping technique as before, listen to
your Transmitted signal without modulation, on another receiver and tap the radio as before.
If you hear a loud noise on your TX signal, the TX side of the vco is also microphonic.
The other sure way to check, is to measure the voltage on the VCO output control voltage pin.
Whilst tapping, check to see that the voltage is stable. If it varies with the tapping, then the VCO is microphonic.
The only cure is to re fix the substrate to the metal housing, either by resoldering it yourself, (good luck),
or you can try this with a hot air gun if your game.
By far the better option though Mr.Rumbling, is to take it to a competent technician who is experienced at these
type of repairs. The result will be a wonderfully performing radio once again, which even today, is still pretty
pager free, and basically bullet proof; and remember, these repairs work on all the 900 series radios, regardless
of the band.
(Lee VK3GK & Dr.Fixit)
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Subject: Making my FM92 sound better. (transmit audio)
Date: Sat Jan 18, 2003
OK, after some fiddling about I think I now have a nice (well OK at least) sounding FM92.
I modelled the preamp in Electronics Workbench and ran some bode plots with and without the 4.7uF electrolytic
capacitor (c3) in the mic preamp.
With the electo, the preamp boosts treble and cuts base. ASCII art time...
|
| /
| __-
| _----
| /
|/
+-----+------+
100 1kHz 10kHz
at 100Hz, the gain is about 8db
at 1kHz, the gain is about 12db
at 10kHz, the gain is about 21db
the -3db point (9db gain) is at about 110Hz
the +3db point (15db gain) is at about 3.6kHz
This explains the 'tinny' sound
Without the electro, you get a reasonably flat response with 12db gain.
|
| ___------
| /
|/
|
+-----+------+
100 1kHz 10kHz
at 100Hz, the gain is 8.7db
at 1kHz, the gain is 12db
at 10kHz, the gain is 12db
the -3db point (9db) is at about 110Hz
So to make an FM92 sound better...
1. Remove the 4.7uF electrolytic capacitor from the mic preamp.
2. adjust R365 (mic sensitivity) on the synthesiser board about 90 degrees, or until it sounds about right
anticlockwise with the PCB in place, or clockwise from the component side.
Good luck.
73s
Mal VK2TMF
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NOTE: only licensed Amateur Radio operators can operate this modified equipment on assigned Amateur frequencies/bands.